Liquidation is CBP's final computation of duty on an entry, usually months after release. Until it happens, the entry is still, in practical terms, open: a PSC filed through your broker in ACE can correct classification, value, origin related data, and other summary errors, with duty differences paid or refunded through the correction itself. This is the cheap window, and importers waste it by not reviewing entries while it is open, because nothing about a quiet entry feels urgent.
After liquidation, a PSC is no longer available and the route is a protest under the customs protest statute, generally within 180 days of liquidation, arguing the legal basis for the change and asking CBP to reliquidate. A protest is a legal filing, not a data fix. It needs the reasoning a PSC does not: why the original treatment was wrong in law, with the authorities cited. Classification protests in particular live or die on that argument.
Clerical and data errors caught early are PSC work: the wrong code keyed, the quantity from the wrong packing list, the exchange rate applied on the wrong date. Errors of judgement discovered late are protest candidates: a classification position you now believe was wrong, a valuation treatment that missed an element. Errors that produced a loss of revenue and were found by you, before CBP, belong in the prior disclosure conversation with counsel, because a correction that quietly pays duty may not limit penalty exposure the way a disclosure does.
Some things neither route fixes casually. Origin claims under a free trade agreement need the underlying certification and evidence to have existed and to be producible. Antidumping scope questions are not clerical. Know which kind of wrong you are before choosing the form, and ask your broker to say it in those terms.
Sample your own entries monthly: pull the ACE reports, compare a sample against commercial invoices, packing lists, and the product master file, and log every discrepancy with its correction route and date. Ten entries a month, honestly reviewed, catch the systemic errors, the supplier who changed origin, the new product keyed to a sibling's code, while PSCs can still repair a whole population of entries sharing the error. The same discovery after liquidation is a protest project and a disclosure decision.
Keep the correction log. It is simultaneously your reasonable care evidence, your broker performance record, and the first document an audit will be glad you have. A broker who files your corrections promptly and reports error patterns back to you is doing the job. A broker whose entries you never review is a hope, not a control.
A successful protest reliquidates the entry and refunds or bills the difference, on CBP's timeline, which is patient. Denied protests on legal questions can be taken to the Court of International Trade within the statutory window after denial. Price that path before you start it: litigation is for precedents and large populations, not for one entry's duty difference, however annoying the principle feels. Group the entries, total the exposure, then decide which fight the number justifies.
A post summary correction must be filed before the entry liquidates. Liquidation timing varies, so the safe practice is to review and correct promptly after entry rather than working to a deadline you do not control.
Generally 180 days from the date of liquidation. Miss it and the liquidation is final for most purposes, which is why importers who track liquidation dates, or whose brokers do, keep a remedy that others lose by default.
You can pay and correct, but the penalty protection of a voluntary prior disclosure generally requires disclosure before CBP notifies a formal investigation of the matter. Once CBP is involved, take trade counsel before filing anything.
Your team owns it. The importer of record is liable for entry accuracy, so an internal sample review, with the broker filing corrections, is the structure that satisfies reasonable care. Reviewing nothing and trusting the broker is the structure audits criticise.